Steam cracking of hydrocarbons is a petrochemical process that is widely used to produce olefins such as ethylene, propylene, C4 olefins (1-butene, 2-butenes, isobutene), butadiene, and aromatics such as benzene, toluene, and xylene. 2-Butenes include cis-2-butene and/or trans-2-butene. In an olefin plant, a hydrocarbon feedstock such as naphtha, gas oil, or other fractions of whole crude oil is mixed with steam. This mixture, after preheating, is subjected to severe thermal cracking at elevated temperatures in a pyrolysis furnace. The cracked effluent from the pyrolysis furnace contains gaseous hydrocarbons of great variety (from 1 to 35 carbon atoms per molecule). This effluent contains hydrocarbons that are aliphatic, aromatic, saturated, and unsaturated, and may contain significant amounts of molecular hydrogen. The cracked product of a pyrolysis furnace is then further processed in the olefin plant to produce, as products of the plant, various individual product streams such as hydrogen, ethylene, propylene, mixed hydrocarbons having four or five carbon atoms per molecule, and pyrolysis gasoline.
Crude C4 hydrocarbons can contain varying amounts of n-butane, isobutane, C4 olefins, acetylenes (ethyl acetylene and vinyl acetylene), and butadiene. See Kirk-Othmer Encyclopedia of Chemical Technology, online edition (2008). Crude C4 hydrocarbons are typically subjected to butadiene extraction or butadiene selective hydrogenation to remove most, if not essentially all, of the butadiene and acetylenes present. Thereafter the C4 raffinate (called raffinate-1) is subjected to a chemical reaction (e.g., etherification, hydration, or dimerization) wherein the isobutene is converted to other compounds (e.g., methyl tert-butyl ether, tert-butyl alcohol, or diisobutene) (see, e.g., U.S. Pat. Nos. 6,586,649 and 4,242,530). The remaining C4 stream containing mainly n-butane, isobutane, 1-butene and 2-butenes is called raffinate-2. Paraffins (n-butane and isobutane) can be separated from the linear butenes (1-butene and 2-butenes) by extractive distillation. Linear butenes can react with ethylene to produce propylene through double-bond isomerization and metathesis reactions (Appl. Ind. Catal. 3 (1984) 215). For example, a mixture of magnesium oxide and silica-supported tungsten oxide can be used for the above transformation to produce propylene.
In a commercial plant, the catalyst tends to deactivate with time, possibly due to the formation of coke in the catalyst pores and on the catalyst surface. Therefore, the catalyst needs to be regenerated periodically. U.S. Pat. No. 4,605,810 teaches a method for regenerating a mixed bed of magnesium oxide and WO3-on-silica by flowing air at 600° C. followed by a nitrogen flush at 600° C. for about 15 minutes, optionally a carbon monoxide flow for at 600° C., and finally a nitrogen flush to cool the catalyst to the desired reaction temperature. However, the present inventor found that such a regeneration method cause significant loss of the catalyst strength, particularly the magnesium oxide.
Magnesium oxide itself is known to be useful as an olefin double-bond isomerization catalyst (see, e.g., U.S. Pat. Nos. 4,217,244 and 5,134,103).
Methods for regenerating MgO-containing catalysts are known. U.S. Pat. No. 3,962,126 teaches a method for reactivating a carbonized magnesium oxide catalyst that has become carbonized when it is in a phenol alkylation reaction, which comprises burning carbon from the catalyst by exposing the catalyst to heat in an oxygen containing gas, to form a partially reactivated catalyst, the improvement which consists essentially of contacting the partially reactivated catalyst with a sufficient amount of water at a temperature below 300° C. to restore the activity of the catalyst.
U.S. Pat. No. 4,217,244 describes a regeneration method of a olefin isomerization catalyst containing magnesium oxide. The regeneration involves purging the catalyst with an inert gas, and then treating the catalyst with an oxygen-containing gas at a temperature not to exceed about 1000° F. (538° C.).
U.S. Pat. No. 5,134,103 discloses a regeneration method of a spent magnesium oxide isomerization catalyst that involves calcining the catalyst at 425 to 590° C.
U.S. Pat. Appl. Pub. No. 2003/0004385 teaches decoking a deactivated magnesium oxide catalyst with a flowing gas containing a dry inert gas (e.g., nitrogen) and an oxidizing agent (e.g., oxygen) at a temperature of at least about 500° C. to substantially completely remove all coke from the catalyst. The regeneration is preferably carried out in steps of gradually increasing temperature and oxygen concentration.